EU social affairs commissioner Marianne Thyssen told Mr O'Leary at a meeting in Brussels on Wednesday that EU rules on employment of air crews were based on where workers left in the morning and returned in the evening - and not where aircraft were registered.

"Respecting EU law is not something over which workers should have to negotiate, nor is it something which can be done differently from country to country," Ms Thyssen said.

"The internal market is not a jungle - it has clear rules on fair labour mobility and worker protection. This is not an academic debate, but about concrete social rights of workers."

However, Coby Benson, a lawyer specialising in flight delay compensation at Bott and Co, said Ryanair's arguments did not comply with the precedent set in April by a case in Germany.

Last month, Ryanair pilots across Europe staged a co-ordinated 24-hour strike to push their demands for better pay and conditions, plunging tens of thousands of passengers into transport chaos at the height of the summer holiday season.

Another indication of the company's rethink on contracts came on Thursday when it announced two new bases in France. They will be the first in the country since it closed Marseille in early 2011 after being sued for employing French workers on Irish contracts.

It will also open another base at Bordeaux for summer 2019 and had another four under consideration.

Two aircraft will be based at both Marseille and Bordeaux and will offer a total of 64 routes and handle 3.5 million passengers a year.